The present invention provides for a process for recovering and recycling helium and unreacted chlorine from the drying and consolidation of optical fiber preforms.
Optical fiber manufacturing is basically a two-phase process that involves fabrication of a specially constructed glass rod called a preform and then melting the preform and drawing it into a thin fiber. Preform fabrication normally involves two steps, deposition and consolidation, that may be combined as one continuous operation or split into two separate ones.
Helium gas has three primary uses in optical fiber manufacture, a carrier gas in preform deposition, a sweep gas in preform consolidation and a heat transfer medium for fiber drawing. Each of these three process steps introduces different impurities, contaminant levels and/or heat levels into the helium gas. The traditional once-through helium flows (i.e. entering the general gas waste stream) used in optical fiber manufacturing processes are wasteful and result in excessive consumption and unnecessarily high cost.
Other consolidation processes, such as disclosed in U.S. Pat. No. 5,055,121, for producing glass preform, has fluorine selectively added to its cladding for optical fiber. This can lower the refractive index of the quartz glass without affecting transmission characteristics of the optical fiber. The glass preform is produced by the steps of deposition of soot of quartz glass on a pipe; dehydration; and vitrification and addition of fluorine.
Dehydration gases include chlorine and chlorine-containing compounds such as SOCl2 and CCl4. In the vitrification and fluorine addition step, fluorine-containing gases such as SF6, CCl2F2, CF4, C2F6 and SiF4 are employed. To obtain the transparent glass preform containing no residual bubbles, helium is the preferred carrier gas for both dehydration and fluorine-addition steps as it is easily dissolved in the glass. Table I summarizes the gas flow rates and concentrations used in the production of glass preform as per the example of the '121 patent.
TABLE IDehydrationFluorine AdditionCl20.6 l/min (6%)SiF40.3 l/min (3%)He 10 l/min (94%)He 10 l/min (97%)
A considerable portion of the chlorine and fluorine-containing gases may leave this process untreated and are currently abated by scrubbing with an alkaline solution. The helium exiting the process is released into the atmosphere. Helium is a non-renewable gas and is expensive. As such, it is highly desirable to recover and recycle the helium to reduce the cost of optical glass fiber production. The recovery of chlorine is desirable from both a cost aspect and an environmental one as well.